Sink roll assembly bearings used for providing tension on a metal strip, being advanced in a bath of molten metal, have a very short life. The short life exists because of distortion and misalignment created between the bearing components, operating in high metal temperatures. In addition, chemical reactions occur between the molten metal and the bearings. Heavy surface unit loading combined with the low operating rotational speed experienced by the rolls, also shortens bearing life.
I have solved some of the problems related to these environmental conditions. An example, may be found in my U.S. Pat. No. 6,692,689 issued Feb. 17, 2004 for “Sink Roll Assembly with Forced Hydrodynamic Film Lubricated Bearings and Self-Aligning Holding Arms”. This invention substantially extended the life of the rolls by aligning the roll shaft with the bearing axis, and pumping molten metal under pressure to the bearing surfaces in the form of a hydrodynamic lubricating film, created by the reduction of the surface unit load acting on the bearing.
The invention disclosed in this application provides a longer operating life by providing ceramic elements in the bearing liner that supports the shaft. Ceramic has a greatly reduced wear pattern compared to the steel alloys normally used for sink rolls, because of its hardness, low coefficient of friction and resistance to chemical attack by the molten metal.
One approach for using ceramic bearing materials has been to use a ceramic liner in a steel alloy sleeve for supporting the shaft. This approach has worked in the laboratory, but is not suitable for an industrial application for several reasons. One reason is that the steel holding ring that carries the ceramic has a much larger coefficient of expansion than the ceramic. The result is that when the ceramic liner is heated to the operating temperature (900° F. to 1300° F. in most cases), the interface between the ceramic liner and the holding sleeve develops a substantial clearance which then causes the unsupported ceramic to shatter. An unsupported ceramic liner will shatter in an industrial application, because of the severe pounding it experiences, caused by the moving metal strip submerged in the molten metal bath.
I have found that employing a steel liner for supporting several ceramic elements in slots in a holding sleeve, prevents the ceramic elements from being separated from the liner and compensates for the differences in the coefficients of expansion of the steel and the ceramic, minimizing the high temperature running clearance.
In the preferred embodiment, the liner has an internal cylindrical surface, having elongated slots in a pattern that extends around the cylindrical surface. An elongated strip of ceramic material is inserted into each of the slots. The collective inner surface of the ceramic elements combines with the liner surface to form a shaft-supporting structure. This arrangement provides the advantage of using the long operating life of a ceramic component without the differential expansion problems.
The process for making such a bearing comprises forming a steel alloy liner having slots for receiving the ceramic elements. The slots, at room temperature, are slightly smaller than the ceramic elements. The liner is then heated to a suitable temperature, such as 1000° F. to expand the slots. The ceramic elements are inserted in the slots. The steel liner is then cooled so that the ceramic elements are tightly held in the slots (i.e. 0.0002±0.0001 inches interference fit). The outside and inside diameters of the steel liner-ceramic inserts assembly are then ground to appropriate dimensions, the outside diameter to receive the holding sleeve, the inside diameter to receive the sink roll shaft trunion.
To prevent the ceramic elements from moving radially outwardly from the liner when operating at high temperature, a holding sleeve is heated to expand it. The heated holding sleeve telescopically receives the liner and the ceramic elements to form a tight fitting connection between the holding sleeve and the steel liner. The holding sleeve and the liner are made of the same materials so that they have the same coefficient of expansion. Thus, as the bearing is heated to an operating temperature, the liner and the holding sleeve expand as a unit while retaining the ceramic elements in their respective slots.
The radial forces applied to the bearing by the shaft are directed toward a particular area of the bearing surface. Over a period of time the shaft will gradually wear the liner in the localized operating area of the bearing surface. The bearing can then be rotated (about 90°) to permit an unworn area of the bearing surface to support the shaft. This can be repeated several times by progressively rotating the bearing whenever required, thus improving by a factor of four the already extended life of the bearing, resulting in additional savings in bearing materials as well as the time in replacing a worn bearing.
Still further objects and advantages of the invention will become readily apparent to those skilled in the art to which the invention pertains upon reference to the following detailed description.